igCM F.H.C.CRICK 



RNA, since polv U H a angle chain without secondary structure. If poly A 

 is added to polv U. to form a double or triple helix, the combination is 

 inactive. Moreover there is preliminary evidence 9 which suggests that sec- 

 ondarv structure within a polynucleotide inhibits the power to stimulate 

 protein synth 



It has vet to be shown bv direct biochemical methods, as opposed to the 

 indirect genetic evidence mentioned earlier, that the code is indeed a triplet 



:npts have been made from a study of the changes produced by muta- 

 tion to obtain the relative order of the bases within various triplets, but my 

 own view is that these are premature until there is more extensive and more 

 reliable data on the composition of the triplets. 



Evidence presented bv several groups 8 - 911 suggest that poly U stimulates 

 both the incorporation ot phenvlalanine and also a lesser amount of leucine. 

 The meaning ot this observation is unclear, but it raises the unfortunate 

 possibilitv ot ambiguous triplets: that is, triplets which may code more than 

 one amino acid. However one would certainly expect such triplets to be in 

 a minoritv. 



It would seem likely, then, that most of the sixtv-four possible triplets will 

 be grouped into twenty groups. The balance of evidence both from the cell- 

 tree system and from the study of mutation, suggests that this does not occur 

 at random, and that triplets coding the same amino acid may well be rather 

 similar. This raises the main theoretical problem now outstanding. Can this 

 grouping be deduced from theoretical postulates? Unfortunately, it is not 

 difficult to see how it might have arisen at an extremelv early stage in evolu- 

 tion by random mutations, so that the particular code we have may perhaps 

 be the result of a series of historical accidents. This point is of more than 

 abstract interest. It the code does indeed have some logical foundation then it 

 is legitimate to consider all the evidence, both good and bad, in any attempt 

 to deduce it. The same is not true if the codons have no simple logical connec- 

 tion. In that case, it makes little sense to guess a codon. The important thing 

 is to provide enough evidence to prove each codon independently. It is not 

 yet clear what evidence can safely be accepted as establishing a codon. What 

 is clear is that most of the experimental evidence so far presented falls short 

 of proof in almost all cases. 



In spite of the uncertainty of much of the experimental data there are 

 certain codes which have been suggested in the past which we can now 

 reject with some degree of confidence. 



b-140 



